What do you mean spend 10$ for test boards? The setup cost is usually several hundred dollars.
also, any reason you don't have an internal ground plane? I'm only familar with a 4 layer, 1 Vcc, 1 Gnd setup, where typically the signals are routed on the top and bottom only if at all possible, and the middle two layers are reserved for ground and power. The unused portions of the bottom layer can then be flooded with ground plane as well to facilitate heat transfer, especially around the QFN. the top layer can be flooded with the 1.2V plane or mini-planes
I think it may be very important to have internal ground plane that the QFN is directly connected to.
I can order 10pcs, 5cm x 5cm boards for $10 plus shipping. But if I throw in some more stuff the the shipping is free. Min. $50 order for free shpg. From China.
I guess originally I had the GND on the bottom as I'd planned to have an open surface for maximum heat sink contact, but later I changed to just having pads for the heat contact and then never revisited moving the GND inside. The internal signal layer will actually have very few traces and will be flooded with GND as well, so it should achieve the same thing even though I haven't called it that. It does now seem an advantage to have the signals visible on the bottom so I could change that.
You can put the thermistor circuit in copper. Populate it on the test board and compare them. I'm working on a project now where we found the temp sensor in the micro to be wildly inaccurate. We had to go with a discrete circuit instead.
If we find the micro to be accurate enough we don't have to populate the external circuit.
Not a bad idea. Will look at that.
I just read a paper from TI recommending 1.0mm spacing.
Do you have a reference for that so I can read up more?